1. Field of the Invention
This invention relates to optical switches and is particularly concerned with switching optical signals composed of light of predetermined wavelengths, for example, Wavelength Division Multiplexed (WDM), Dense WDM (DWDM), or Coarse WDM (CWDM) optical signals used in optical telecommunications.
2. Background Art
Optical transmission systems achieve their end-to-end connectivity by concatenating multiple spans between intermediate switching nodes. When the end-to-end granularity of any given transmission path is a fraction of the capacity of a given optical carrier, time division multiplexing (TDM) protocols are applied, which share the overall bandwidth of a carrier signal. In this case, the individual signals (tributaries) are switched electronically at the intermediate nodes, since individual tributaries can only be accessed by demultiplexing the TDM signal.
On the other hand, Wavelength Division Multiplexing (WDM), and particularly DWDM and CWDM transmission can provide manifold capacity expansion on existing fibre links. DWDM optical networks transmit multiple channels (wavelengths) on each optical fiber in the network. The result is a plurality of channels on each fiber, a channel carrying information between two terminals in the networks. An advantage of the WDM networks is that conversions between the optical and electrical domains take place practically only at the periphery of the transport network. The signals are add/dropped and amplified within the network in optical format.
However, current WDM optical networks typically convert channel signals into electrical signals at every switching node in the network because optical switches having sufficiently large enough port counts are not available, nor is optical reach sufficient. Conversion is performed using transmitters (Tx), receivers (Rx), transceivers (Tx-Rx pair) or transponders at every port of the switching node, and for every channel. (Transponders are devices that convert the signal between the optical and electrical domains, and also translate the wavelength of the channels at the border between the long and short reach networks.)
These converters are expensive. As the number of channels carried by an optical fiber increases, the required accuracy of the converters also increases, and hence the cost. Moreover, as the number of ports per switching node increases, the required number of converters also increases. Consequently, large networks carrying dense DWDM signals require many costly converters and are therefore costly to build.
There is a substantial advantage in designing optical transmission networks such that the majority of the channels (wavelengths) can be routed end-to-end via optical switches and optical amplifiers, without the use of converters (e.g. transponders) on a per channel wavelength basis at intermediate sites or nodes. This leads to a need for an optical cross-connect switch optimized for routing wavelengths from end to end, as opposed to a large opaque optical switch fabric placed between banks of transponders.
There are proposals to build large, purely optical switches that offer full connectivity between all their ports. However, fabrication of these large optical switches has proven difficult. Currently, large non-blocking optical switches use a large number of switch modules. One example of this envisages building a 128 port×128 port switch out of three stages of multiple 16×16 crosspoint matrices, or a 512×512 port switch out of three stages of multiple 32×32 crosspoint matrices, in a three stage CLOS architecture. The above is based on the availability of 16×16 or 32×32 switch matrices in the form of Micro-Electro-Mechanical (MEM) switch matrices (described in e.g. “Free-space Micromachined Optical-Switching Technologies and Architectures”, Lih Y. Lin, AT&T Labs-Research, OFC99 Session W14-1, Feb. 24, 1999).
Other multi-stage approaches use smaller matrices and more stages. Even the 3 stage CLOS architecture is limited to 512–1024 switched wavelengths with 32×32 switch matrix modules, which, in today's 160 wavelength per fiber DWDM environment, is only adequate to handle the output/input to 3 fiber pairs (480 wavelengths). In addition, current multi-stage switches have significant problems, even at three stages. These problems include high overall optical loss through the switch, since the losses in each stage are additive across the switch, and there is the potential for additional loss in the complex internal interconnect between the stages of the switch. Size limitations in terms of the number of wavelengths switched can be overcome by going to a five stage CLOS switch, but this further increases the loss through the switch as well as it adds to its complexity and cost. In addition, a CLOS switch requires a degree of dilation (i.e. extra switch paths) to be non-blocking and each optical path has to transit three (or five) individual modules in series.
MEM mirrors technology has evolved lately. The ‘3-D MEMS’ devices have emerged as the photonic switch technology of choice for large fabric switches. 3-D MEMS is a term used by the Applicant for a mirror mounted on a frame that can be rotated along two axes, giving it four degrees of freedom. The 3-D MEMS devices are arranged preferably in a matrix, which comprises besides the mirrors a control system for positioning the mirrors independently.